Thread-forming screws typically are screws having a thread adapted to produce a counter or female thread in a bore of a workpiece. Usually, one differentiates between a tapping and forming thread of a screw. In case of a tapping thread a counter thread is produced in a preformed hole by a chip-generating forming process. In case of a self-forming thread the counter thread is generated substantially without chips through a plastic deformation of the workpiece. In this connection it is known to provide self-forming screws with a thread-generating zone by which particularly effectively a counter thread can be produced. If the workpieces are made of plastic material, the preformed hole or the preformed bore normally has not the core or root diameter of the thread-forming screw, rather, is provided with a greater diameter, preferably with the thread pitch diameter of the thread-forming screw.
For the improved insertion of the thread-forming screw in the preformed bore, the thread-forming screw normally is provided with a first cut or a conical portion by which a centering in the bore is achieved and an easy moving of the threads of a thread-forming portion into the wall of the bore, as well.
Threaded connections with thread-forming screws suffer under undesired release, for example by vibrations. Frequently, such thread-forming screws are pre-assembled with corresponding workpieces, e.g. of synthetic material. In this case, the thread-forming screw is partially turned into the bore of the workpiece. During transportation of such pre-assembled units vibration may occur which may lead to a loosening of the screws from the workpieces. It is known to undertake respective provisions, e.g. by applying micro capsules on the thread. The micro capsules effect an increased friction and thus may avoid an undesired loosening of the screw in the bore. However, the application of such micro capsules is relatively expensive.